The present invention is directed to a fire detector which is capable of converting various quantity levels of a detected object (heat, smoke, light and so on) into a predetermined number of fire-stage informations.
The specification of British Pat. No. 1,478,952 discloses an electric circuit for an ion smoke detector or a photoelectric smoke detector, having an output section adapted to deliver two outputs, i.e. a high and a low output, through the switching of the resistance value of a threshold component between low and high values by means of a switching circuit which is constructed such that the operation thereof is limited by a combination of Zener diodes for switching the resistance of the switching circuit between a high and low level depending on the level of voltage applied thereto.
Even when a multiplicity of fire detectors dispersed in a fire prevention zone deliver only two outputs to a central signal receiving device, it is physically impossible to detect a fire occurring in the whole area of the zone or to observe the safe condition of the entire zone with the above described conventional arrangement.
Japanese Patent Laid-open Publication No. 64297/1977 discloses a fire detector in which the operation level of the detector is switched between two stages periodically by an oscillator which is located in the detector.
In the type of fire detector which is adapted to periodically switch the sensitivity over two stages, in order that the receiver can distinguish the fire signals corresponding to the operation levels, the fire signal of one operation level is in the form of pulse signals, while the fire signal of another operation level is in the form of a continuous signal. Therefore, under a condition such that the continuous signal is being delivered as a result of detecting an increasing temperature or smoke density caused by a fire, the fire signal coming from another detector connected to the same detector line cannot be distinguished the signal from the above-described other detector, when the another detector starts to operate. It is impossible to know the number of operating detectors in a detector line, unless an independent signal system is used. The use of the independent signal system, however, renders the detector system complicated and expensive.